The present invention relates to a lubricant composition used as lubricating oil or grease and a bearing lubricated with the lubricant composition. More particularly, the present invention relates to the lubricant composition capable of preventing a flaking phenomenon from occurring in an early stage with a rolling surface of the bearing turning into white in its texture. The present invention also relates to the bearing lubricated with the lubricant composition.
A bearing lubricated with gear oil or hydraulic oil is used in a hydraulic motor, a hydraulic pump, an axle planetary gear part. As the oil, mineral oil and water-glycol hydraulic oil are used.
In recent years, because the use condition of the bearing lubricated with the gear oil and the hydraulic oil has become severe. For example, the bearing is rotated at a high speed and a high load is applied thereto. Consequently the flaking phenomenon occurs in an early stage with the rolling surface of the bearing turning into white in its texture. The present invention also relates to the bearing lubricated with the lubricant composition.
Unlike flaking which occurs in the interior of the rolling surface owing to metal fatigue, the flaking phenomenon that occurs in an early stage in the rolling surface at a comparatively shallow portion thereof with the rolling surface of the bearing turning into white in its texture. That is, the flaking phenomenon is a hydrogen embrittlement-caused destruction phenomenon. The hydrogen causing the flaking to occur is generated by decomposition of lubricant oil. The hydrogen is generated from the lubricating oil for the following two reasons: (1) decomposition owing to heat and shear generated on the rolling surface and (2) decomposition reaction which takes place with a metal surface generated by wear acting as a catalyst. The generated hydrogen penetrates into bearing steel easily, thus causing the flaking to occur owing to the hydrogen embrittlement. Which of (1) and (2) affects the generation of the hydrogen to a higher extent depends on a use condition of the bearing. When the generation of the hydrogen is caused mainly by (2), reducing the time period in which the fresh metal surface generated by wear is exposed is effective for restraining the generation of the hydrogen. That is, adding an oxidizing agent or the like which makes the generated fresh metal surface inactive immediately to the lubricating oil is effective for restraining the generation of the hydrogen.
Based on the above-described knowledge, the following measures for preventing the rolling surface from flaking owing to the hydrogen embrittlement are proposed: As an example, an oxide film is formed on the rolling surface by using iron oxide black treating method (disclosed in Japanese Utility Model Application Laid-Open No. 6-43349). As another example, a passivating agent is added to grease to form an oxide film on a wear-caused surface generated on the rolling surface (disclosed in Japanese Patent Application Laid-Open No. 3-210394). As still another example, Cr is contained in the material of the bearing ring to form an oxide film on the rolling surface to prevent flaking (U.S. Pat. No. 3,009,254) from occurring.
However, in the oxide film disclosed in Japanese Utility Model Application Laid-Open No. 6-43349, it is necessary to immerse the bearing ring in a water solution of caustic soda heated to a low temperature (130 to 160° C.) to form an iron oxide black (Fe3O4) on the rolling surface of the bearing in advance. Thus the manufacturing process is complicated. Each of the above-described conventional arts has a disadvantage that the oxide film formed on the rolling surface may flake therefrom owing to a frictional wear between the oxide film and the rolling element. Thus satisfactory measures have not been taken so far.
As disclosed in Japanese Patent Application Laid-Open No. 3-210394, the oxide film is formed on the bearing ring of the bearing. As also disclosed in U.S. Pat. No. 3,009,254, the grease for lubricating the bearing contained the substance for accelerating oxidation of the rolling surface. However, the effective formulation is not clear on the measure for the flaking devised by giving attention to the lubricating oil for use in the bearing. Thus the measures have been hardly used so far.
Year by year, electric parts and auxiliary machines of a vehicle and motors of industrial machines are demanded to be compact and have a high performance and output. Hence the use conditions thereof have become severe. For example, in electric parts and auxiliary machines used in an engine room, reduction of output caused by compactness is compensated by rotating them at high speeds. A rolling bearing is used for them. Grease is mainly used to lubricate them. However, because the use conditions have become severe, the rolling surface of a rolling bearing flakes in an early stage with the rolling surface of the bearing turning into white in its texture. This is a problem to be solved.